1. Field of the Invention
The present invention pertains to the detection of water in a chamber in which a vacuum is being drawn. It is particularly useful in chemical vapor sterilization techniques.
2. Background
In many instances having water in a vacuum chamber during the application of a vacuum is undesirable. The problem is of particular concern in chemical vapor sterilization techniques in which a chamber is drawn below atmospheric pressure.
A typical chemical vapor sterilization cycle begins by cleaning and drying the instruments to be sterilized and placing them into a chamber. The chamber is heated and the atmosphere in the chamber is evacuated. After achieving a strong vacuum, the vapor phase sterilizing agent is introduced into the chamber, either directly as a vapor or as a mist which quickly vaporizes in the vacuum. The vapor bathes the instruments killing bacteria, viruses and spores on the surfaces of the instrument in contact with the vapor. Hydrogen peroxide, ethylene oxide, and chlorine dioxide, among others, are suitable sterilants. A particularly advantageous system employs hydrogen peroxide vapor in connection with a gas plasma. The following U.S. Patents, incorporated fully herein by reference, describe such processes in more detail: U.S. Pat. Nos. 4,643,876 issued Feb. 17, 1987 to Jacobs et al. and 4,756,882 issued Jan. 27, 1987 to Jacobs et al.
To ensure that the hydrogen peroxide vapor will penetrate into cracks, crevices and particularly long lumens and the like in the instruments undergoing sterilization, air and water vapor in the chamber are evacuated prior to releasing the hydrogen peroxide vapor into the chamber. After the chamber is evacuated, the chemical vapor enters the chamber. The added vapor in the chamber slightly raises its pressure and the chemical vapor rushes to equalize the pressure throughout the chamber thereby quickly entering lumens and the like.
Water in the chamber inhibits complete permeation of the chamber, especially tight spaces, and full contact of the instruments with the chemical vapor through several mechanisms. Water vaporizing within the chamber dilutes the chemical vapor. In addition, if the water molecules have a higher diffusivity than the chemical vapor they will more efficiently reach the tight spaces, thereby reducing the concentration of the chemical vapor therein. Thus, water in the system can reduce the overall sterilization efficiency. Water initially present in the system as vapor will be removed as the system is drawn into vacuum. However, water initially present as liquid may vaporize either during the application of vacuum or afterwards to form water vapor present in the system. Water must vaporize to be eliminated from these systems.
Liquid water initially present in the system may cause additional problems by freezing as the vacuum is drawn. As a vacuum is drawn on the chamber, liquid water therein begins to evaporate as the total pressure in the chamber is decreased to the vapor pressure in the liquid. The liquid to vapor phase change requires heat and thus the water gives up its heat to evaporation and cools. When the water has cooled sufficiently, it freezes. The resulting ice particles may locally inhibit contact of the chemical vapor with the instrument, or in more severe cases may block narrow passageways. In nearly all sterilization methods, including hydrogen peroxide and hydrogen peroxide/gas plasma, operators know to check for the presence of liquids following the procedure, and if any liquids are detected the load is dried and the procedure repeated. Accordingly, it has long been desired to have some method of detecting the presence of liquid in a load to be sterilized prior to actually performing the sterilization process.